1. Field of the Invention
The present invention is directed to the field of determining the appropriate time to change lubricating oil in internal combustion engines and more specifically to an on-board method and apparatus for directly measuring the change in the engine oil condition.
2. Description of the Prior Art
Currently, in many automotive applications, only the pressure of the lubrication system during engine operation or the level of the oil within the sump prior to engine operation are measured on-board a vehicle. The necessity of changing oil is normally communicated to the vehicle operator via the operator's manual and is based upon the measured use of the vehicle rather than the actual condition of the oil. That adherence to the estimated deterioration of the oil based upon measured use of the vehicle (mileage) is adequate for most engine or vehicle usage. However, when an inferior grade of oil is substituted or the engine is operated in harsh environments the proper interval for oil replacement should be considerably shortened to avoid engine wear. Accordingly, it is desirable to provide an on-board system which directly measures a performance-related property of the oil and provides an indication when the usefulness of the oil as an engine lubricant has deteriorated past a certain level.
U.S. Pat. No. 4,443,754 describes a method for determining the minimum lubricating oil-film thickness within an operating engine by measuring electrical capacitance. In that patent, one of the main bearings of the engine is electrically isolated from the engine block and a capacitance signal is developed between the crankshaft and the isolated bearing so that the entire bearing acts as one plate of a capacitor and the crankshaft acts as the other plate of a capacitor. The degradation of the oil to the point of having a minimum film thickness allows the two plates of the capacitor to become closer together and the electrical capacitive properties to change. The decreased thickness is most likely due an increase in viscosity which, in turn, prevents the oil from flowing between the plates of the capacitor. A circuit is employed which allows the capacitor to be pulsed and monitored until the minimum thickness level is detected.
European Patent Application Publication No. 80632 describes an apparatus that detects the degree of deterioration of lubricating oil by measuring the dielectric constant of the lubricating oil. In that apparatus, a pair of spaced-apart electrodes are immersed in lubricating oil to form a sensor capacitor, the capacitance of which varies as a function of the dielectric constant of the lubricating oil. The sensor capacitor is placed in the oil flow system of the engine so that the oil will flow between the plates of the capacitor. The sensor capacitor is electrically connected in series with another capacitor having a fixed capacitance value to form a voltage divider. AC voltage applied across the divider is divided at a ratio determined by the dielectric constant of the lubricating oil so that the AC voltage across the sensor capacitor is substantially proportional to the engine oil dielectric constant.
European Patent Application Publication No. 121739 also describes a method and apparatus for performing the evaluation of dielectric substances such as lubricating oil. In that publication, an electrical capacitive device having at least a pair of electrodes is disposed in contact with the dielectric substance and a power supply is used for apply a pulse voltage to the electrodes. A current detector is connected to the other side of the capacitor for detecting a transient response current flowing between the electrodes that is dependent upon the component of the dielectric substance disposed between the electrodes. A signal processor is used for evaluating the performance of the dielectric substance based upon the peak value of the transient response current.